How bad is it to use someone else's toothbrush? even if this person is your significant other?

Try to avoid: if they have periodontal disease there is possibility you can get some of these disease causing bacteria With brushing you are depositing these bacteria right into your gums Don't share

Personal hygiene: Since you've probably shared just about every germ with your significant other, there's no new germs to pass. The question is, why do you have to share? Blood and food debris can be stuck between the bristles, and two people using one brush justs makes the brush wear out faster, necessitating more frequent replacement. They're cheap enough to splurge on each of you having one.